In microelectronic applications, polymers that demonstrate high temperature resistance are generally well known. Precursors of such polymers, such as polyimides and polybenzoxazoles, can be made photoreactive with suitable additives. The precursors are converted to the desired polymer by known techniques such as exposure to high temperatures. The polymer precursors are used to prepare protective layers, insulating layers, and relief structures of highly heat-resistant polymers.
Conventional positive-working photosensitive polybenzoxazoles (PBO) contain an alkaline soluble PBO precursor and a diazoquinone photoactive compound as shown in U.S. Pat. No. 4,371,685. The diazoquinone compound inhibits the solubility of the PBO precursor in an aqueous base. After exposure to light, the diazoquinone compound undergoes photolysis and converts to indenecarboxylic acid, which promotes the aqueous base solubility of the PBO precursor. Positive-working photosensitive formulations based on an alkaline soluble PBO precursors capped with diazoquinone moieties and diazoquinone photoactive compounds are recently disclosed in U.S. Pat. Nos. 6,177,225 B1 and 6,127,086.
Cured PBO formulations are typically dark in color because of the myriad of high temperature reactions that take place during the high temperature cure. Unfortunately, the color of cured films of the positive photosensitive formulations is important for alignment during wire bonding and other packaging and assembly operations. Lighter-colored, more transparent films offer significant advantages in alignment during wire bonding and other packaging and assembly operations. This can result in increased throughput and accuracy. Japanese Kokai 09-146273 discloses a method to suppress such darkening in which an additional exposure step after development is employed to flood exposing the previously unexposed areas. This incurs additional costs, results in slower throughput and may affect some of the curing properties of the composition.
It is an object of this invention to provide photosensitive compositions capable of curing to high temperature resistant features, which will not substantially darken during typical cure temperatures. We have surprisingly found that positive polybenzoxazole precursor photosensitive formulations containing diazoquinone photoactive compounds which lack benzylic hydrogens on the PAC backbone yield cured films significantly lighter in color. Previous patents disclosing positive polybenzoxazole photosensitive formulations using diazoquinone photoactive compounds which lack benzylic hydrogens on the PAC backbone did not disclose any report about the color of the resulting cured films of such formulations nor how to address the problem of dark cured film in PBO formulations. Such formulations are disclosed in U.S. Pat. No. 5,376,499, U.S. Pat. No. 5,726,279, U.S. Pat. No. 5,783,654, U.S. Pat. No. 5,883,221, U.S. Pat. No. 5,922,825, U.S. Pat. No. 5,973,202, U.S. Pat. No. 6,120,970 or U.S. Pat. No. 6,153,350 herein incorporated by reference.